1. Field of the Invention
The invention relates to an antenna mirror scanner method and apparatus, and more particularly relates to an antenna mirror scanner method and apparatus for use in remote earth radiometric sensors.
2. Description of the Prior Art
A conventional method for generating conically scanned narrow antenna beams includes feeding or exciting a rotating tilted planar reflector, i.e., a splash plate, with a non-rotating direct-aperture source, such as a horn or a parabolic feed antenna. The axis of symmetry of the non-rotating direct-aperture antenna is aligned with a spin axis of the planar reflector. The rotation of the tilted planar reflector about its spin axis directs the resulting antenna beam in a narrow planar or conical scan. When the tilt angle of the tilted planar reflector is 45.degree., that is, when the plane of the planar reflector is tilted with respect to the spin axis, a planar (i.e., crosstrack) scan is achieved. When the tilt angle is at an angle other than 45.degree., a conical scan is produced.
Radiometers detect and measure radiant electromagnetic radiation. Conventional high spatial resolution radiometers typically employ narrow planar or conically scanned antenna beams such as those described previously for remote earth sensing of thermal noise emissions. A planar reflector included within the radiometer may be excited by radiant energy, i.e., thermal noise emissions, incident upon a reflective planar surface of the rotating planar reflector. The radiant energy is detected and measured from a portion of the earth tracked by a directed scan of the antenna mirror scanner.
One example of a conventional antenna mirror scanner utilizing a rotating planar reflector is NASA's advanced microwave precipitation radiometer 2 shown in FIG. 1. The advanced microwave precipitation radiometer 2 functions as a remote earth radiometric sensor, including a planar reflector 4 which rotates about a spin axis 10. Thermal emission radiation incident upon reflective surfaces 9 and 11 of planar reflector 4 is directed through either of horn focusing lenses 5 and 7 to non-rotating direct-aperture antennas 6 and 8, respectively. A pressure lid 12 encloses encoder and scanner control electronics 14 and a calibration load section 16. The spin axis 10 of the rotating planar reflector 4 is aligned with each axis of symmetry of the non-rotating direct-aperture antennas 6 and 8.
The symmetrical axes of non-rotating direct-aperture antennas 6 and 8 are positioned such that they are parallel to the spin axis of the planar reflector 4. Because of the parallel alignment of the axes, the advanced microwave precipitation radiometer 2 rotates the polarization vectors, i.e., the resultant electric and magnetic fields of the detected radiation, as the planar reflector 4 rotates about its spin axis 10 to direct the detected radiation to non-rotating direct-aperture antennas 6 and 8.
The rotation of the polarization vectors of the antenna scanner beam using the conventional mirror scanner methods such as that described above is a major disadvantage. For example, a satellite-borne radiometer scanning the earth to detect thermal noise emissions using the conventional mirror scanner method will require extensive and complex processing interpretation of the thermal noise radiation data when the antenna polarization vectors rotate as a function of a locus of the scan.